Unlike general mail system, mail is read in web browser without particular interface or protocol for receiving mail from mail server in web mail system. In initial state of web mail, small size storage space, for example 5 megabytes (MB) was given in web mail service. However, recently large size storage space up to 1 gigabyte (GB) is being provided. Data having similar size with text data was transmitted as attachment file in the past, however, recently moving picture file or MP3 file more than tens of MB can be transmitted through web mail. Therefore, the uses of web mail are being changed. That is, web mail was mainly used for text sending in the past. However, large size web mail service enabled mail service to be used in file storage and management of document.
Recently, various mails such as good order verification mail, delivery verification mail are transmitted as electronic commerce through internet increases. However, most of mails related to the electronic commerce were sent by automatic mail sending system, and therefore same mail title was given for each of category. Therefore, users should open the mail if the users want to check goods related to the mail. Besides the electronic commerce, many users use email for management of file and document. However, users cannot know to which file or document a mail is related, unless users open the mail.
In prior art, preview function is provided for users to check mail contents without opening mail in web mail service as well as mail browser program such as OUTLOOK EXPRESS®. However, in order to perform preview, the web mail system should transmit mail contents along with title of each mail to the subscriber terminal, by which load of mail server definitely increases. Load of mail server occurs in local mail server as well as the web mail server.